1. Field of the Invention
The invention in general relates to monitoring and diagnostic equipment, and particularly to equipment for measuring blade tip displacement of selected blades in one or more blade rows of a turbo machine.
2. Description of the Prior Art
Turbines or similar machines include one or more blade rows each including a plurality of radially extending blades connected to a rotating shaft member. A typical blade is of a complex design which results in multiple vibrational modes. If the natural resonance of the blade in one or more of these modes should coincide with the machine's rated rotational speed, or harmonics thereof, then the blade will have a tendency to vibrate to an extent where the blade's tip will oscillate excessively about its normal position. When the amplitude of this oscillation exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery so that the problem may be corrected.
Accordingly, the blades should be tested for excessive vibration prior to being put into operation in the field so that any required design changes may be implemented prior to the machine going on line.
For those machines which are already in operation, monitoring of the blade vibration is extremely important since different pressure and temperature conditions can change the blade's resonance. In addition, there is a possibility of non-resonant vibration, known as flutter, which, if excessive, can also lead to stress fracture.
One widely used method which tests for excessive blade vibration utilizes various sensors such as strain gages affixed to the rotating blades. Sensor information is communicated to analyzing equipment outside the machine by means of miniature transmitters affixed to the machine's rotating shaft at various locations.
Although the arrangement provides for highly accurate results, the method is limited in that only a certain portion of the blades can be tested at any one time due to the limited number of transmitters that can be accommodated inside the machine. To test all of the blades therefore requires the machine to be shut down each time a new group of blades is to be tested so that the sensors may be properly affixed. The cost of the transmitters, and even the batteries therefore, are abnormally high since they must be of special design to withstand the extremely hostile environment inside the turbine. The high cost of equipment, in addition to labor costs, make such testing method prohibitive for many plant operators.
To obviate such high costs, another testing method utilizes permanently installed, non-contacting proximity sensors to detect blade tip movement. Many of these systems determines the position of a blade under test, from a reference point to see whether or not it deviates from a known position by a predetermined amount. Still other systems determine whether or not the distance between two blades varies at several positions during the course of rotation of the blades.
The apparatus of the present invention is of the type which utilizes a plurality of non-contacting sensors and in which compensation is provided in the event of sensor misalignment. This compensation, coupled with a novel detection arrangement results in a system which is highly reliable, extremely precise, and gives accurate results.